Fluid Properties (College Board AP® Physics 1: Algebra-Based)

Fluid properties

What is a fluid?

• Liquids and gases are collectively known as fluids

• Unlike solids, these have weak interactions between particles

  • In a liquid, particles are weakly bonded, allowing the substance to flow

  • In a gas, particles are not bound together, so they can spread out freely

• Fluids have no fixed shape

  • Liquids fill the base of a container

  • Gases expand to fill a container completely

• Most questions will assume that a fluid is ideal

• An ideal fluid is incompressible

  • This means that force applied to one region of a fluid is exerted on other regions of the fluid

• An ideal fluid has a viscosity of zero

  • This means resistive forces do not need to be considered in questions involving flowing ideal fluids

Fluid density

What is density?

• Density is defined as:

The mass per unit volume of a substance

• The density of an ideal fluid can be a characteristic property

  • This is because an ideal fluid is incompressible, so the density remains constant

Density equation

• The density of a fluid can be calculated using:

• Where:

  • = density of the fluid, measured in

  • = mass of the fluid, measured in

  • = volume of the fluid, measured in

• This equation appears on the equation sheet

Measuring density

Measuring the density of regular objects

• The volume of a regular object can be calculated accurately by taking measurements of its dimensions

  • Common examples are cubes, cylinders, and spheres

• A procedure to find the volume of a regular object is as follows:

  1. Place the object on a digital balance and note down its mass

  2. Use either a ruler, callipers, or a micrometer to measure the object’s dimensions (width, height, length, radius) – the measuring apparatus will depend on the size of the object

  3. Repeat these measurements and take an average of these readings

  4. Use these average values and a suitable formula for the regular shape to calculate its volume

  5. Substitute the mass and volume into the density equation

Measuring the density of irregular objects

• The density of an irregularly shaped object can be found using the displacement method

  • This method requires the object to displace a volume of water

  • A beaker with a spout pointing downwards, called a displacement (or eureka) can, is required

• A procedure to find the volume of an irregular object is as follows:

  1. Place the object on a digital balance and note down its mass

  2. Fill the displacement can with water up to a point just below the spout

  3. Place an empty measuring cylinder below its spout

  4. Carefully lower the object into the eureka can

  5. Measure the volume of the displaced water in the measuring cylinder

  6. The volume of water displaced is equal to the volume of the object

  7. Repeat these measurements and take an average before calculating the density

Equipment to measure the density of irregular objects